Satellites and space probes generally include photovoltaic generators for powering on-board equipment and for charging batteries that deliver power during periods of eclipse. The photovoltaic generators, the batteries, and the various items of equipment that need to be powered are connected to one another by a power supply bus bar that presents a potential that needs to be kept within a predetermined range. Regulators are provided for controlling the magnitude of the currents delivered by said photovoltaic generators, in particular as a function of the potential of said power supply bus bar.
The regulator in the most widespread use is the sequential switching shunt regulator (S3R or S3R) as developed by the European Space Agency and as described in the article “The sequential switching shunt regular S3R” by D. O'Sullivan and A. Weinberg, Proceedings of the Third ESTEC Spacecraft Power Conditioning Seminar, Noordwijk, the Netherlands, Sep. 21-23, 1977. That regulator comprises a plurality of individual photovoltaic generators connected in parallel to feed a power supply bus bar. Each individual generator can be selectively short-circuited by a controlled switch, and under such circumstances the current it generates is no longer supplied to the bus bar, but is dissipated; the various short-circuit switches are switched ON or OFF as a function of the potential of the power supply bus bar, and with the help of hysteresis comparators having thresholds that are offset from one another. Thus, the lower the potential of the bus bar, indicative of high consumption by the equipment being powered and/or of a low level of charge in the batteries, the greater the number of individual generators that are connected to said bus bar. Conversely, when the potential of the bus bar is high, generators are short-circuited, which means that there is excess power available.
The S3R regulator constitutes an excellent compromise between the requirements for effectiveness and for simplicity, however it does not make it possible to optimize the use of the power available from the photovoltaic generators. Such generators present a V-I characteristic curve that presents an optimum operating point at which the power extracted is maximized; in order to operate at this optimum operating point, each generator must be connected to a load that presents a determined input impedance. However the problem is made much more complex by the fact that the characteristic curves vary very greatly with aging of the generators, and also depend strongly on temperature. That is why, if it is desired to make optimum use of the available power, which is very important in particular for interplanetary missions directed to the outer regions of the solar systems, it is necessary to provide a control system that makes it possible to “track” the maximum power operating point, with this being known as maximum power point tracking (MPPT).
In conventional manner, MPPT systems use microprocessor-based controllers, but that is generally not desirable in space applications, in particular for reasons of reliability. That is why analog MPPT controllers have been developed in the past, but until now none of them has given complete satisfaction.
The articles “Electrical power subsystem of Globastar” by W. Denzinger, Fourth European Space Power Conference 1995, and “Power conditioning unit for Rosetta/Mars Express” by H. Jensen and J. Laursen, Proceedings of the Sixth European Space Power Conference 2002, describe systems based on the principle that at the maximum power point, the absolute value of the dynamic impedance dV/dI of a generator is equal to its static impedance V/I. This makes it possible to avoid multiplying voltage and current values, and consequently to limit the complexity of the circuit. Nevertheless, circuit complexity remains excessive for providing independent control of a large number of individual generators. Thus, a plurality of solar arrays need to be connected in parallel and controlled together; this limits the effectiveness of the system and makes it necessary to use protection and isolation systems in order to avoid faults propagating.
U.S. Pat. No. 4,794,272 discloses an MPPT system that makes use of a different concept, namely maximizing the output current from a DC/DC switching converter (an array power regulator (APR)) connected between the photovoltaic generators and the power supply bus bar. This simplification is possible because the output voltage from the APR converter is equal to the potential of the power supply bus bar, which can be considered as being approximately constant. Nevertheless, such a system requires means for modifying the operating points in order to cause it to oscillate about its maximum value, thereby preventing the complexity of the electronic circuit from being reduced significantly.
U.S. Pat. No. 6,316,925 discloses another MPPT system that controls a voltage converter in order to maximize the output voltage by using operations of sampling and comparing output current values. Such a system presents the drawback of being synchronous (the sample-and-hold circuit is driven by a clock), thereby limiting its performance. Good tracking of the optimum operating point can be obtained only by using slow oscillations around said optimum point, but that implies a long acquisition time in order to come close thereto.
Furthermore, the above two documents disclose in conventional manner, the use of a buck converter (a voltage-reducing switching converter) as the APR converter. That presents three major drawbacks:                firstly, it is necessary to use high voltage solar generators (operating at several tens of volts (V) and up to more than 100 V), thus running a major risk of failure due to electric arcs forming in operation:        secondly, a buck switching converter includes a controlled switch (typically a metal oxide on silicon field effect transistor (MOSFET)) connected in series with each generator; as a result, in the event of the switch failing, the generator is permanently isolated from the power supply bus bar;        and thirdly, when the switch of a buck switching converter is constituted by a MOSFET, the MOSFET must be controlled by a floating driver circuit, thereby increasing the number of electronic components that are needed.        
The article by A. Boehringer and J. Haussmann entitled “Dynamic behavior of power conditioning systems for satellites with a maximum power point tracking system”, published in the Proceedings of the Spacecraft Power Conditioning Electronic Seminar, ESTEC Noordwijk, the Netherlands, July 1972, describes an MPPT system based on the above-mentioned “dV/dI=V/I” concept and using a step-up switching converter operating at the limit between discontinuous and continuous conditions.